The objective of this investigation is to determine the origin, evolution and epidemiology of the R plasmids of the virulent, antibiotic-resistant strains of Staphylococcus aureus that have been involved in outbreaks of staphylococcal infections. The R-plasmids from these strains will be transferred to strains lacking plasmids and from these strains R-plasmids will be purified and characterized by electron microscopy for their contour lengths and molecular weights and by their pattern of electrophoresis following restriction endonuclease digestion. The possible role of the common plasmids in the evolution of penicillin plasmids will be determined. Resistance plasmids of Staphylococcus epidermidis will be isolated and similarly characterized to show their molecular relatedness to R-plasmids of S. aureus. By heterologous DNA-DNA hybridizations and electron microscopy of heteroduplexes the possible role of the R-plasmids of S. aureus and S. epidermidis in the origin of R plasmids of these two strains will be investigated.